Die-down ball grid array package with die-attached heat spreader and method for making the same

ABSTRACT

An apparatus, system, and method for assembling a ball grid array (BGA) package is described. A stiffener/heat spreader is provided. A substrate has a first surface and a second surface. The substrate has a central window-shaped aperture that extends through the substrate from the first substrate surface to the second substrate surface. The first substrate surface is attached to a surface of the stiffener/heat spreader. A portion of the stiffener/heat spreader is accessible through the central window-shaped aperture. An IC die has a first surface and a second surface. The first IC die surface is mounted to the accessible portion of the stiffener/heat spreader. A drop-in heat spreader has a surface that is mounted to the second IC die surface.

CROSS-REFERENCE TO OTHER APPLICATIONS

The following application of common assignee is related to the presentapplication, and is herein incorporated by reference in its entirety:

“Enhanced Die-Up Ball Grid Array and Method for Making the Same,” Ser.No. 09/742,366, filed Dec. 22, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of integrated circuit (IC)device packaging technology, and more particularly to substratestiffening and heat spreading techniques in ball grid array (BGA)packages.

2. Related Art

Integrated circuit (IC) dies are typically mounted in or on a packagethat is attached to a printed circuit board (PCB). One such type of ICdie package is a ball grid array (BGA) package. BGA packages provide forsmaller footprints than many other package solutions available today. ABGA package has an array of solder balls located on a bottom externalsurface of a package substrate. The solder balls are reflowed to attachthe package to the PCB. The IC die is mounted to the package substrate.Wire bonds typically couple signals in the IC die to the substrate. Thesubstrate has internal routing which electrically couples the IC diesignals to the solder balls on the bottom substrate surface.

A number of BGA package substrate types exist, including ceramic,plastic, and tape (also known as “flex”). In some BGA package types, aheat spreader/stiffener may be attached to the substrate to provide heatsinking, and to supply planarity and rigidity to the package.

Die-up and die-down BGA package configurations exist. In die-up BGApackages, the IC die is mounted on a top surface of the substrate orheat spreader/stiffener, on a side opposite that of the solder balls. Indie-down BGA packages, the IC die is mounted on a bottom surface of thesubstrate or stiffener, which is the same side as the solder balls.

The tape substrate used in flex BGA packages is typically polyimide,which has very low values of thermal conductivity. Consequently, the ICdie is separated from the PCB by the tape substrate thermal barrier. Thelack of direct thermal connection from IC die to PCB leads to relativelyhigh resistance to heat transfer from IC die to printed circuit board(theta-jb).

Furthermore, conventional BGA packages are subject to high thermalstresses that result from the heat given off during operation of themounted IC die. The thermal stresses are primarily imposed on the IC diedue to a mismatch of the thermal expansion coefficient (CTE) between thesemiconductor die and the stiffener/heat spreader. The thermal expansioncoefficient (CTE) of copper typically used for a heat spreader in a tapeBGA package is approximately 17.4×10⁻⁶/°C. For a silicon IC die, the CTEis approximately 2.64×10⁻⁶/°C. Because of the difference in CTE values,changes in temperature during the BGA package assembly process may leadto high levels of thermal stress. As the IC die size increases for a BGApackage, higher stress levels will occur at the interface of the IC dieand stiffener/heat spreader. Consequently, cracks often occur on largesemiconductor IC dies during the portions of the assembly processfollowing the attachment of the IC die to the stiffener/heat spreader.

Hence, what is needed are BGA packages with improved heat spreadingcapabilities. What is also needed is a reduction in BGA package thermalstress during the assembly process, to improve packaging yields.

SUMMARY OF THE INVENTION

The present invention is directed to an apparatus, system, and methodfor assembling a ball grid array (BGA) package. In one aspect, astiffener/heat spreader is provided. A substrate has a first surface anda second surface. The substrate has a central window-shaped aperturethat extends through the substrate from the first substrate surface tothe second substrate surface. The first substrate surface is attached toa surface of the stiffener/heat spreader. A portion of thestiffener/heat spreader is accessible through the central window-shapedaperture. An IC die has a first surface and a second surface. The firstIC die surface is mounted to the accessible portion of thestiffener/heat spreader. A drop-in heat spreader has a surface that ismounted to the second IC die surface.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1A illustrates an example die-down BGA package.

FIG. 1B illustrates a heat transfer path from an IC die to a PCB for theexample die-down BGA package of FIG. 1A.

FIG. 2A illustrates a cross-sectional view of a BGA package, accordingto an embodiment of the present invention.

FIG. 2B shows a perspective view of an example drop-in heat spreader,according to an embodiment of the present invention.

FIG. 2C illustrates a bottom view of the BGA package of FIG. 2A,according to an embodiment of the present invention.

FIG. 3A illustrates a cross-sectional view of a BGA package thatincludes an exemplary drop-in heat spreader, according to an embodimentof the present invention.

FIG. 3B shows a perspective view of the drop-in heat spreader shown inFIG. 3A, according to an embodiment of the present invention.

FIG. 3C illustrate a bottom view of the BGA package of FIG. 3A, withwire bonds from an IC die to a drop-in heat spreader, according to anembodiment of the present invention.

FIG. 3D illustrates a cross-sectional view of BGA package that includesan exemplary drop-in heat spreader with a portion protruding through anencapsulant, according to an embodiment of the present invention

FIG. 4 illustrates a bottom view of an exemplary solder ball arrangementfor a BGA package.

FIG. 5 shows an example layer of a substrate, with routing and vias.

FIG. 6 shows a flowchart providing operational steps for assembling oneor more embodiments of the present invention.

The present invention is described in the following with reference tothe accompanying drawings. In the drawings, like reference numbersindicate identical or functionally similar elements. Additionally, theleft-most digit(s) of a reference number identifies the drawing in whichthe reference number first appears.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Overview

The present invention is directed to a method and system for improvingthe mechanical, thermal, and electrical performance of BGA packages. Thepresent invention is applicable to all types of BGA substrates,including ceramic, plastic, and tape (flex) BGA packages. Furthermorethe present invention is applicable to die-up (cavity-up) and die-down(cavity-down) orientations. Numerous embodiments of the presentinvention are presented herein.

Preferably, the invention is directed to heat sinking techniques indie-down BGA packaging. A top surface of an IC die is attached to apackage stiffener/heat spreader in a die-down BGA package. A drop-inheat spreader is attached to a bottom surface of the IC die. Byattaching the top and bottom surfaces of the IC die to heat spreaders, asymmetrical support structure surrounding the IC die is created. Thissupport structure is substantially more symmetrical than the packageheat spreader/IC die combination of conventional die-down BGA packages.

An advantage of the configuration of the present invention is a reliefof, or a balancing of, stress that may bend an IC die in a conventionaldie-down BGA package. A conventional die-down BGA package may beconsidered to be similar to a “bi-metal” system. When temperature rises,the stiffener/heat spreader-to-IC die combination bends in a directionof the material with lower value of CTE; towards the IC die. Whentemperature is lowered, the heat spreader/IC die combination bends in adirection of the material with higher value of CTE; towards the heatspreader. The present invention described herein substantially forms a“tri-metal” system, with the IC die sandwiched between two heatspreaders. Preferably, the package stiffener/heat spreader ismanufactured from the same material as the drop-in heat spreader. Insuch an arrangement, the package stiffener/heat spreader-to-ICdie-to-drop-in heat spreader combination will not bend significantlywith a change of temperature. This is because with temperature changes,both heat spreaders will bend towards or away from the IC die,essentially canceling each other's bending motion.

In a further embodiment, the drop-in heat spreader may be used as eithera power plane or a ground plane for the die-down BGA package, to improvedie-down BGA package electrical performance. IC die power or ground padsmay be wire bound to a drop-in heat spreader attached to the center ofthe IC die. The drop-in heat spreader may be exposed at the bottom ofthe die-down BGA package encapsulation. The exposed surface of thedrop-in heat spreader may be attached to the PCB. The drop-in heatspreader may be attached to a power or ground potential in the PCB. Suchan arrangement may allow for the reduction of power/ground traces andpower/ground solder balls, and may lead to shorter distance for supplyand return current. These shorter distances may reduce undesiredinductances.

Ball grid array package types are described below. Further detail on theabove described embodiments, and additional embodiments according to thean present invention, are presented below. The embodiments describedherein may be combined in any applicable manner, as required by aparticular application.

Ball Grid Array (BGA) Package

A ball grid array (BGA) package is used to package and interface an ICdie with a printed circuit board (PCB). BGA packages may be used withany type of IC die, and are particularly useful for high speed ICs. In aBGA package, solder pads do not just surround the package periphery, asin chip carrier type packages, but cover the entire bottom packagesurface in an array configuration. BGA packages are also referred to aspad array carrier (PAC), pad array, land grid array, and pad-grid arraypackages. BGA packages types are further described in the followingparagraphs. For additional description on BGA packages, refer to Lau, J.H., Ball GridArray Technology, McGraw-Hill, New York, (1995), which isherein incorporated by reference in its entirety.

Die-up and die-down BGA package configurations exist. In die-up BGApackages, the IC die is mounted on a top surface of the substrate orstiffener, which is the side opposite that of the solder balls. Indie-down BGA packages, the IC die is mounted on a bottom surface of thesubstrate or stiffener, which is the same side as the solder balls.

A number of BGA package substrate types exist, including ceramic,plastic (PBGA), and tape (also known as “flex”). FIG. 1A illustrates atape BGA package 100. Tape BGA package 100 includes an IC die 102, atape substrate 104, a plurality of solder balls 106, one or more wirebonds 108, a package stiffener/heat spreader 110, and one or morestiffener wire bonds 122. Tape or flex BGA packages are particularlyappropriate for large IC dies with large numbers of input and outputs,such as application specific integrated circuits (ASIC) andmicroprocessors.

Tape substrate 104 is generally made from one or more conductive layersbonded with a dielectric material. For instance, the dielectric materialmay be made from various substances, such as polyimide tape. Theconductive layers are typically made from a metal, or combination ofmetals, such as copper and aluminum. Trace or routing patterns are madein the conductive layer material. Substrate 104 may be asingle-conductive-layer (single-layer) tape, a two-conductive-layer(two-layer) tape, or additional conductive layer tape substrate type. Ina two-layer tape, the metal layers sandwich the dielectric layer, suchas in a copper-Upilex-copper arrangement. Substrate 104 has a centralwindow-shaped aperture 112 to accommodate IC die 102, as furtherdescribed below.

Package stiffener/heat spreader 110 may be laminated to substrate 104.Stiffener 110 is typically made from a metal, or combination of metals,such as copper, tin, and aluminum, or may be made from a polymer, forexample. Stiffener 110 also acts as a heat sink, and allows for heatspreading in BGA package 100. Stiffener 110 has a central cavity 114 onits bottom surface. Stiffener 110 may be configured in other ways thanshown in FIG. 1A. For example, in other configurations, the bottomsurface of stiffener 110 may not include a central cavity 114, and mayinstead be flat.

IC die 102 is attached directly to stiffener 110, for example, by anepoxy 134. IC die 102 is any type of semiconductor integrated circuit,which has been formed in, and separated from a semiconductor wafer.

One or more wire bonds 108 connect corresponding bond pads 118 on IC die102 to contact points 120 on substrate 104. Bond pads 118 are coupled tosignals internal to IC die 102, including logical signals, and power andground potentials. Furthermore, one or more wire bonds 122 may connectcorresponding bond pads 118 to contact stiffener points 126 on stiffener110. For instance, stiffener 110 may be used as a power or ground plane.

An epoxy or encapsulant 116 covers IC die 102 and wire bonds 108 and 122for mechanical and environmental protection.

As described above, BGA package 100 includes an array or plurality ofsolder balls 106 located on a bottom external surface of packagesubstrate 104. IC die 102 is electrically connected to substrate 104 byone or more wire bonds 108. Wire bonds 108 are electrically connected tosolder balls 106 on the bottom surface of substrate 104 throughcorresponding vias and routing in substrate 104. Vias in substrate 104can be filled with a conductive material, such as solder, to allow forthese connections. Solder balls 106 are attached to substrate 104, andare used to attach BGA package 100 to a PCB.

Note that although wire bonds, such as wire bonds 108, are shown anddescribed herein, IC dies may be mounted and coupled to a substrate withsolder balls located on a surface of the IC die, by a process commonlyreferred to as “C4” or “flip chip” packaging.

FIG. 4 illustrates a bottom view of an exemplary solder ball arrangementfor BGA package 100. FIG. 4 shows a 12 by 12 array of solder balls onthe bottom surface of substrate 104. Other sized arrays of solder ballsare also applicable to the present invention. Solder balls 106 arereflowed to attach BGA package 100 to a PCB. The PCB may include contactpads to which solder balls 106 are bonded. PCB contact pads aregenerally made from a metal or combination of metals, such as copper,nickel, tin, and gold. The solder ball array may be organized in anynumber of ways, according to the requirements of the particular BGApackage application.

As described above, the BGA package substrate provides vias and routingon one or more layers to connect contact pads for wire bonds to solderballs attached to the bottom substrate surface. FIG. 5 shows an examplelayer 502 of substrate 104, with routing and vias, for accomplishingthis. A plurality of circular metal ball pads 504 are shown. Metal ballpads 504 may be patterned from a metal layer in substrate 104. Asdescribed above, substrate 104 includes central window-shaped aperture112. Central window-shaped aperture 112 is preferably aligned, butlarger than central cavity 114. A plurality of conductive metal traces506 are shown coupled to metal ball pads 504, and ending in metalfingers 508 near the periphery of central window-shaped aperture 112. Insome cases, metal traces 506 may be connected to a power ring 510. Aplurality of vias 512 are also shown.

The present invention is applicable to improving thermal and electricalperformance in the BGA package types described herein, and further BGApackage types. FIG. 1B illustrates a heat transfer path from IC die 102to a PCB 128 for BGA package 100. BGA package 100 provides a conductivethermal path from an active surface 130 of IC die 102, to a back surface132 of IC die 102, to a die attach epoxy 134, to stiffener/heat spreader110, to a tape attach epoxy 136, to substrate 104, to plurality ofsolder balls 106, to PCB 128. Arrows 138 show directions of heat flowfrom IC die 102 to PCB 128.

As shown in FIG. 1B, a zone 140 between IC die 102 and PCB 128 does notsignificantly contribute to heat transfer from IC die 102 to PCB 128,even though providing a relatively short distance between IC die 102 andPCB 128. The lack of heat transfer through zone 140 may be attributed tolow conduction heat transfer coefficients of encapsulant 116 (the “globtop”) and an air gap 142 between encapsulant 116 and PCB 128.

As shown in FIG. 1A, stiffener/heat spreader 110 may be used as a groundplane to reduce ground inductance. To further reduce inductance andresistance for power distribution, a separate power ring, such as powerring 510 shown in FIG. 5, must be constructed on substrate 104. Traceshave to be added to the routing of substrate 104 to connect between thepower ring and corresponding solder ball pads. A disadvantage of thisconfiguration is an increased complexity in the design of substrate 104,and a reduced space for the routing of signal traces in substrate 104.Furthermore, power inductance increases with an increase in trace lengthin substrate 104.

BGA Package Drop-in Heat Spreader Embodiments According to the PresentInvention

Further details of structural and operational implementations of thepresent invention are described in the following sections. Thesestructural and operational implementations are described herein forillustrative purposes, and are not limiting. The invention as describedherein may be implemented in both die-up and die-down BGA package types,as well as in additional IC package types. Furthermore, each of theembodiments presented below are applicable to tape substrate BGApackages, as well as BGA packages with alternative substrate types,including BT, FR4, and ceramic, to name a few. One such package that mayinclude aspects of the present invention is a die-up BGA package,developed by Broadcom Corp., which is located in Irvine, Calif. Thedescription below is adaptable to these and other package types, aswould be understood to persons skilled in the relevant art(s) from theteachings herein.

Features of each of the embodiments presented below may be incorporatedinto BGA packages independently, or may be combined in any manner in aBGA package, as would be apparent to persons skilled in the relevantart(s) from the teachings herein.

According to embodiments of the present invention, a heat spreader maybe used in a BGA package to provide for thermal stress relief to thepackage, enhance dissipation of heat from the package, and improvepackage electrical performance. In a preferred embodiment, a drop-inheat spreader is attached to the bottom surface of an IC die in adie-down flex BGA package to provide one or more of these advantages.

FIG. 2A illustrates a cross-sectional view of a die-down flex BGApackage 200, according to an embodiment of the present invention. BGApackage 200 includes IC die 102, substrate 104, plurality of solderballs 106, one or more wire bonds 108, package stiffener/heat spreader110, encapsulant 116, one or more stiffener wire bonds 122, and adrop-in heat spreader 202. FIG. 2C illustrates a bottom view of die-downflex BGA package 200, according to an embodiment of the presentinvention. Encapsulant 116 is not shown in FIG. 2C.

Substrate 104 has a top surface to which a bottom surface of stiffener110 is mounted. A bottom surface of substrate 104 attaches the pluralityof solder balls 106. The plurality of solder balls 106 connect to viasand/or points on the bottom surface of substrate 104 to which signalsinternal to substrate 104 are routed and exposed. Substrate 104 includescentral window-shaped aperture 112 that extends through substrate 104from the top substrate surface to the bottom substrate surface.

A portion of stiffener/heat spreader 110 is accessible through centralwindow-shaped aperture 112 of substrate 104. In an embodiment, centralcavity 114 in stiffener 110 is accessible through window-shaped aperture112. Central cavity 114 in stiffener 110 coincides with centralwindow-shaped aperture 112 in substrate 104, to accommodate IC die 102.A top surface of IC die 102 is mounted in central cavity 114 ofstiffener 110.

In an alternative embodiment, stiffener 110 does not have a centralcavity 114, and has a bottom surface that is substantially planar. In anembodiment, a portion of the substantially planar bottom surface ofstiffener 110 is accessible through window-shaped aperture 112. The topsurface of IC die 102 is mounted to the accessible portion of the bottomsurface of stiffener 110.

In an alternative embodiment, substrate 104 does not have a centralwindow-shaped aperture 112, and is instead continuous. In such anembodiment, the top surface of IC die 102 is mounted to the bottomsurface of substrate 104. Stiffener 110 may be coupled to IC die 102through substrate 104 by one or more solder-filled vias, for instance,for thermal and/or electrical coupling.

One or more wire bonds 108 connect corresponding bond pads 118 on IC die102 to contact points 120 on substrate 104.

In an embodiment, stiffener 110 is configured to operate as a ground orpower plane. For example, one or more stiffener wire bonds 122 connectcorresponding ground bond pads 118 to contact points 126 on stiffener110. Stiffener 110 may be coupled to a ground signal in the PCB throughone or more solder balls 106 and vias and/or routing in substrate 104.

As shown in FIG. 2A, a surface of drop-in heat spreader 202 is attachedto the top surface (active surface) of IC die 102 using an epoxy 204 orsimilar substance. Epoxy 204 may be the same substance as encapsulant116, the same material as epoxy 134, or may be a different substance.Silver filled epoxies may be used for epoxy 204 to enhance heatextraction from IC die 102. Encapsulant 116 may be used for attachmentof the drop-in heat spreader to IC die 102. In an embodiment, whenencapsulant 116 is used to attach the drop-in heat spreader, the heatspreader is placed on the active surface of the IC die surface beforeapplication of the encapsulant.

FIG. 2B shows a perspective view of an example heat spreader 202,according to an embodiment of the present invention. Top surface 206 andbottom surface 208 of heat spreader 202 shown in FIG. 2B aresubstantially rectangular in shape. Heat spreader 202 includes acircumferential surface 210 that extends around heat spreader 202between top surface 206 and bottom surface 208. Heat spreader 202 may beconfigured in other shapes, such as where top surface 206 and bottomsurface 208 of heat spreader 202 are elliptical or round, and othershapes.

The material used for drop-in heat spreader 202 may be one or moremetals such as copper or aluminum, for example. Heat spreader 202 may bemachined, molded, or otherwise manufactured from these materials. Heatspreader 202 may be made from the same material as stiffener/heatspreader 110. In such an embodiment, stiffener/heat spreader 110 anddrop-in heat spreader 202 would have the same thermal expansioncoefficient. In such a configuration, the combination of stiffener/heatspreader 110, IC die 102, and drop-in heat spreader 202 will not bendsignificantly with a change of temperature. As temperature changes, bothstiffener 110 and heat spreader 202 will bend towards or away from ICdie 102, substantially canceling each other's bend.

In an embodiment, heat spreader 202 is configured to operate as a groundplane or power plane. For example, one or more ground or power wirebonds may be used to connect bond pads on IC die 102 to contact pointson heat spreader 202. When bottom surface 208 of heat spreader 202 isexposed after connection to IC die 102, bottom surface 208 can beattached to a PCB using solder, conductive epoxy, or other substances.The PCB connection area is connected to a PCB power or ground plane.Such a configuration may reduce or eliminate power or ground traces onsubstrate 104, and reduce the number of solder balls attached tosubstrate 104 that are dedicated to power or ground. This configurationmay also lead to shorter current travel lengths, and may reduceinductance and resistance related to 13GA package 200. Furthermore, aportion of circumferential surface 210 may also be exposed when bottomsurface 208 is exposed.

A drop-in heat spreader may be shaped to provide for easier connectionof power or ground wire bonds to the drop-in heat spreader. Furthermore,a portion of the drop-in heat spreader may also protrude throughencapsulant 116 for thermal and electrical connection with the PCB. FIG.3B shows a perspective view of a drop-in heat spreader 300, according toan embodiment of the present invention. Heat spreader 300 includes aridge 302. FIG. 3A illustrates a cross-sectional view of BGA package 200that includes heat spreader 300, according to an embodiment of thepresent invention. As shown in FIG. 3B, a first surface 304 and secondsurface 306 of drop-in heat spreader 202 are substantially planar andsubstantially parallel to each other. A circumferential surface 314extends around heat spreader 300 between first surface 304 and secondsurface 306. Ridge 302 extends around at least a portion of thecircumference of heat spreader 202, such that an area of surface 306 isgreater than that of an area of surface 304. A pedestal 312 of heatspreader 300 is formed by a portion of circumferential surface 314 ofheat spreader 300 between ridge 302 and first surface 304.

Ridge 302 provides a convenient connection point for wire bonds. FIG. 3Cillustrates a bottom view of BGA package 200, with wire bonds from ICdie 102 to heat spreader 300, according to an embodiment of the presentinvention. For example, one or more ground wire bonds 308 connectcorresponding ground bond pads 118 on IC die 102 to contact points 310on heat spreader 300. As shown in FIG. 3A, contact points 310 may belocated on ridge 302 of heat spreader 300.

As shown in FIGS. 2A and 3A, heat spreaders 202 and 300 may be smallerin area than the bottom surface of IC die 102. Alternative sizes forheat spreaders 202 and 300 are also applicable to the present invention,including sizes equal to the area of IC die 102, or larger areas. Heatspreaders 202 and 300 are shaped and configured to spread heat from ICdie 102, as is required by the particular application. For example, bymaximizing the size of heat spreader 202 and 300, such that edges ofheat spreaders 202 and 300 are close to wire bond pads on IC die 102,self-inductance due to a reduced wire bond length may be minimized.

An encapsulant may be used to encapsulate the IC die and at least aportion of the drop-in heat spreader. In FIG. 2A, IC die 102 and drop-inheat spreader 202 are completely encapsulated by encapsulant 116. In analternative embodiment, such as shown in FIG. 3A, a portion of drop-inheat spreader 300 (surface 304) is exposed through encapsulant 116. Aportion of the circumferential surface of heat spreader 300 betweensurface 304 and surface 306 may also be exposed through encapsulant 116.For example, all or a portion of pedestal 312 may protrude throughencapsulant 116, as shown in FIG. 3D. As described above, when exposed,the exposed surface of the drop-in heat spreader may be configured to beattached to a PCB. For instance, the exposed surface may be plated withsolder. By exposing a surface of the drop-in heat spreader forattachment to a PCB, a greater transfer of heat from the BGA package maybe obtained.

By attaching drop-in heat spreader 202 or 300 to the top surface of ICdie 102, the mechanical structure of BGA package 200 becomes moresymmetrical in its center region. Thermal stress at the interface of ICdie 102 and stiffener 110 is substantially released or altered by thedrop-in heat spreader. Deformation caused by thermal stress issubstantially reduced through the use of a drop-in heat spreader, suchas heat spreader 202 or 300. Drop-in heat spreaders 202 and 300 allowfor even larger sizes of IC die 102 and greater I/O counts by providingfor greater heat spreading capacity in BGA package 200.

FIG. 6 shows a flowchart 600 providing operational steps for assemblingone or more embodiments of the present invention. For example, the stepsof flowchart may be used to assemble BGA package 200. The steps of FIG.6 do not necessarily have to occur in the order shown, as will beapparent to persons skilled in the relevant art(s) based on theteachings herein. Other structural embodiments will be apparent topersons skilled in the relevant art(s) based on the followingdiscussion. These steps are described in detail below.

Flowchart 600 begins with step 602. In step 602, a substrate is providedthat has a first surface and a second surface, wherein the substrate hasa central window-shaped aperture that extends through the substrate fromthe first substrate surface to the second substrate surface. Forexample, the substrate is tape substrate 104, or another substrate typesuitable for a BGA package. The central window-shaped aperture iswindow-shaped aperture 112.

In step 604, a stiffener/heat spreader is provided. For example, thestiffener is stiffener/heat spreader 110.

In step 606, a surface of the stiffener/heat spreader is attached to thefirst substrate surface, wherein a portion of the stiffener/heatspreader is accessible through the central window-shaped aperture. Forexample, a surface of stiffener 110 is attached to a surface ofsubstrate 104. In an alternative embodiment, substrate 104 does not havea central window-shaped aperture, but is continuous.

In step 608, a first surface of an IC die is mounted to the accessibleportion of the stiffener/heat spreader. For example, the IC die is ICdie 102, which is mounted to stiffener 110. In an alternativeembodiment, when substrate 104 is continuous, the first surface of ICdie 102 is mounted to substrate 104.

In step 610, a surface of a drop-in heat spreader is mounted to a secondsurface of the IC die. For example, the drop-in heat spreader may beheat spreader 202 or 300, which is mounted with epoxy 204 or otherattachment means to the bottom surface of IC die 102. Heat spreader 202or 300 typically is mounted to the center of the bottom surface of ICdie 102, and covers less than the entire bottom surface of IC die 102.For instance, the smaller area of heat spreader 202 or 300 allows forbond pads 118 to be exposed on the bottom surface of IC die 102 for wirebond connections. In alternative embodiments, heat spreader 202 or 300is of the same size, or comprises a larger area than the upper surfaceof IC die 102.

A benefit of performing the steps of flowchart 600 is that the heatspreader relieves thermal stress at an interface of the IC die and thefirst stiffener surface. Further benefits may include an enhancement ofheat dissipation from the BGA package, and an improvement in BGA packageelectrical performance.

In an embodiment, flowchart 600 comprises the additional step where aplurality of solder balls are attached to the second substrate surface.For example, the plurality of solder balls are plurality of solder balls106, which connect to vias and/or solder ball pads on the bottom surfaceof substrate 104. The solder balls may be arranged on the bottom surfaceof substrate 104 as shown in FIG. 4, for example, or in alternativearrangements. The solder balls are used to attach a BGA package to aPCB.

In an embodiment, the second IC die surface includes a contact pad. Forexample, the contact pad may be contact pad 118. Flowchart 600 mayfurther comprise the step where the contact pad is coupled to thedrop-in heat spreader with a wire bond. For example, the wire bond maybe wire bond 108, 122, or 308. In an embodiment, the contact pad is aground contact pad. Flowchart 600 may comprise the additional step wherethe ground contact pad is coupled to the drop-in heat spreader with thewire bond, wherein the drop-in heat spreader operates as a ground plane.

In an embodiment, step 610 comprises the step where a drop-in heatspreader is provided that has a first planar surface, wherein saiddrop-in heat spreader has a second planar surface, wherein said firstand said second planar surfaces are substantially parallel to eachother, wherein said drop-in heat spreader has a ridge around at least aportion of its circumference such that an area of said first planarsurface is greater than that of said second planar surface. For example,the ridge is ridge 302. In an embodiment, flowchart 600 comprises theadditional step where the wire bond is attached to the ridge of thedrop-in heat spreader. For example, the wire bond is wire bond 308.

In an embodiment, flowchart 600 comprises the additional step where asecond surface of the drop-in heat spreader is configured to be attachedto a printed circuit board. For example, the bottom surface of the heatspreader is exposed, and may be plated with solder.

In an embodiment, flowchart 600 comprises the additional step where acentral cavity is formed in the stiffener/heat spreader surface, whereinthe central cavity forms at least a portion of the accessible portion ofthe stiffener/heat spreader. For example, the central cavity is centralcavity 114. In an embodiment, step 608 comprises the step where the ICdie is mounted in the central cavity.

In an embodiment, step 610 comprises the step where a drop-in heatspreader is provided that is substantially planar, wherein theaccessible portion of the stiffener/heat spreader is centrally locatedon the substantially planar stiffener/heat spreader surface.

In an embodiment, step 608 comprises the step where the IC die firstsurface is mounted to the stiffener/heat spreader with a first epoxy,wherein the drop-in heat spreader is mounted to the IC die with a secondepoxy. For example, the first epoxy is epoxy 134, and the second epoxyis epoxy 204.

In an embodiment, an area of the second IC die surface is greater thanan area of a surface of the drop-in heat spreader. Flowchart 600 mayinclude the additional step where the drop-in heat spreader isconfigured to mount to the center of the second IC die surface.

In an embodiment, flowchart 600 comprises the additional step where theIC die and the drop-in heat spreader are encapsulated. For instance, ICdie 102, heat spreader 202 or 300, and wire bond 108 are encapsulated bya molding compound or epoxy, shown as encapsulant 116. In a firstembodiment, the IC die and the drop-in heat spreader are encapsulatedwhere neither are exposed, as shown in FIG. 2A, for example. In a secondembodiment, a surface of the drop-in heat spreader is exposed, as shownin FIG. 3A, for example. In a third embodiment, a portion ofcircumferential surface of the drop-in heat spreader is exposed andprotrudes through the bottom surface of encapsulant 116, as shown inFIG. 3D, for example.

In an embodiment, step 602 comprises the step where a tape substrate isprovided. For example, substrate 104 may be a tape substrate.

In an embodiment, flowchart 600 comprises the additional step where athermal expansion coefficient of the stiffener/heat spreader is matchedto the thermal expansion coefficient of the drop-in heat spreader. Forexample, stiffener 110 and heat spreader 202 or 300 are constructed fromthe same material, such that their thermal expansion coefficients aresubstantially matched.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A ball grid array (BGA) package, comprising: a stiffener; a substratethat has a first surface and a second surface, wherein said substratehas a window-shaped aperture that extends through said substrate fromsaid first substrate surface to said second substrate surface, whereinsaid first surface of said substrate is attached to a surface of saidstiffener, and wherein a portion of said surface of said stiffener isaccessible through said window-shaped aperture; an IC die that has afirst surface and a second surface, wherein said first surface of saidIC die surface is mounted to said accessible portion of said surface ofsaid stiffener; a heat spreader that has opposing first and secondsurfaces, wherein said first surface of said heat spreader is mounted tosaid second surface of said IC die, wherein said heat spreader has aridge between said first and second surfaces of said heat spreader; andat least one wire bond that couples at least one contact pad of said ICdie to said ridge of said heat spreader.
 2. The package of claim 1,further comprising: a plurality of solder balls attached to said secondsurface of said substrate.
 3. The package of claim 1, wherein said heatspreader is configured to dissipate heat generated by said IC die. 4.The package of claim 1, wherein said second surface of said heatspreader is configured to be attached to a printed circuit board.
 5. Thepackage of claim 1, wherein said contact pad is a ground contact pad,and wherein said heat spreader operates as a ground plane.
 6. Thepackage of claim 1, wherein said stiffener includes a central cavity insaid surface of said stiffener, wherein said central cavity forms atleast a portion of said accessible portion of said surface of saidstiffener, wherein said IC die is mounted in said central cavity.
 7. Thepackage of claim 1, wherein said surface of said stiffener issubstantially planar, and wherein said accessible portion of saidsurface of said stiffener is centrally located on said surface of saidstiffener.
 8. The package of claim 1, wherein said IC die is mounted tosaid surface of said stiffener with a first epoxy layer, and whereinsaid heat spreader is mounted to said IC die with a second epoxy layer.9. The package of claim 1, wherein an area of said second surface ofsaid IC die is greater than an area of said first surface of said heatspreader, and wherein said heat spreader is configured to mount to thecenter of said second surface of said IC die.
 10. The package of claim1, wherein said IC die and said heat spreader are encapsulated.
 11. Thepackage of claim 1, wherein said IC die and a portion of said heatspreader are encapsulated by an encapsulant material, wherein saidsecond surface of said heat spreader is exposed through said encapsulantmaterial.
 12. The package of claim 11, wherein said heat spreaderincludes a circumferential surface between said first and said secondsurfaces of said heat spreader, wherein said circumferential surface isat least partially exposed.
 13. The package of claim 1, wherein saidsubstrate is a tape substrate.
 14. The package of claim 1, wherein saidstiffener and said heat spreader have the same thermal expansioncoefficient.
 15. The package of claim 1, wherein said heat spreadercomprises at least one metal.
 16. The package of claim 15, wherein saidat least one metal includes copper.
 17. The package of claim whereinsaid at least one metal includes aluminum.
 18. The package of claim 1,wherein said heat spreader is electrically conductive.
 19. The packageof claim 1, further comprising: an encapsulant material thatencapsulates said IC die and said heat spreader on said second surfaceof said stiffener.
 20. The package of claim 19, wherein said encapsulantmaterial is used to maintain said heat spreader in contact with saidsecond surface of said IC die.
 21. The package of claim 19, furthercomprising: an encapsulant material that encapsulates said IC die and aportion of said heat spreader on said surface of said stiffener.
 22. Thepackage of claim 21, wherein said second surface of said heat spreaderis not encapsulated by said encapsulant material and is accessible. 23.The package of claim 21, wherein said heat spreader includes acircumferential surface between said first and second surfaces of saidheat spreader, wherein said second surface of said heat spreader and aportion of said circumferential surface are not encapsulated by saidencapsulant material and are accessible.
 24. The package of claim 1,wherein said stiffener is a second heat spreader.
 25. The package ofclaim 1, further comprising: a thermally conductive adhesive materialthat attaches said first surface of said heat spreader to said secondsurface of said IC die.
 26. The package of claim 1, further comprising:an electrically conductive adhesive material that attaches said firstsurface of said heat spreader to said second surface of said IC die. 27.The package of claim 1, wherein said heat spreader includes: acircumferential surface around said heat spreader that connects saidfirst and said second surfaces of said heat spreader; and wherein saidridge is opposed to said second surface of said heat spreader in saidcircumferential surface.
 28. The package of claim 1, wherein said heatspreader is substantially rectangular in shape.
 29. The package of claim28, wherein said second surface of said heat spreader is capable ofbeing attached to a contact area on a printed circuit board when the BGApackage is mounted to the printed circuit board.
 30. The package ofclaim 1, wherein said ridge extends around a portion of thecircumference of the heat spreader.
 31. The package of claim 1, whereinsaid ridge extends around the heat spreader.
 32. A system for assemblinga ball grid array (BGA) package, comprising: means for attaching asurface of a stiffener to a first surface of a substrate, wherein thesubstrate has a window-shaped aperture that extends through thesubstrate from the first surface of the substrate to a second surface ofthe substrate, wherein a portion of the stiffener is accessible throughthe window-shaped aperture; means for mounting a first surface of an ICdie to the accessible portion of the stiffener; means for mounting asurface of a heat spreader to a second surface of the IC die; and meansfor coupling a contact pad of the IC die to a ridge of the heat spreaderbetween the first surface and a second surface of the heat spreader witha wire bond.
 33. The system of claim 32, wherein the substrate is a tapesubstrate.
 34. A ball grid array (BGA) package, comprising: astiffener/heat spreader; a substrate that has a first surface and asecond surface, wherein said substrate has a central window-shapedaperture that extends through said substrate from said first substratesurface to said second substrate surface, wherein said first substratesurface is attached to a surface of said stiffener/heat spreader,wherein a portion of said stiffener/heat spreader is accessible throughsaid central window-shaped aperture; an IC die that has a first surfaceand a second surface, wherein said first IC die surface is mounted tosaid accessible portion of said stiffener/heat spreader, wherein saidsecond IC die surface includes a contact pad; a drop-in heat spreaderthat has a surface that is mounted to said second IC die surface; and awire bond that couples said contact pad to said drop-in heat spreader;wherein said first surface of said drop-in heat spreader is planar,wherein said drop-in heat spreader has a second planar surface, whereinsaid first and said second planar surfaces are substantially parallel toeach other, wherein said drop-in heat spreader has a ridge around atleast a portion of its circumference such that an area of said firstplanar surface is greater than that of said second planar surface; andwherein said wire bond attaches to said ridge of said drop-in heatspreader.
 35. A method of assembling a ball grid array (BGA) package,comprising the steps of: providing a substrate that has a first surfaceand a second surface, wherein the substrate has a central window-shapedaperture that extends through the substrate from the first substratesurface to the second substrate surface; providing a stiffener/heatspreader; attaching a surface of the stiffener/heat spreader to thefirst substrate surface, wherein a portion of the stiffener/heatspreader is accessible through the central window-shaped aperture;mounting a first surface of an IC die to the accessible portion of thestiffener/heat spreader, wherein the second IC die surface includes acontact pad; and mounting a surface of a drop-in heat spreader to asecond surface of the IC die, including the step of providing a drop-inheat spreader that has a first planar surface, wherein the drop-in heatspreader has a second planar surface, wherein the first and the secondplanar surfaces are substantially parallel to each other, wherein thedrop-in heat spreader has a ridge around at least a portion of itscircumference such that an area of the first planar surface is greaterthan that of the second planar surface; and coupling the contact pad tothe drop-in heat spreader with a wire bond, including the step ofattaching the wire bond to the ridge of the drop-in heat spreader.
 36. Amethod of assembling a ball grid array (BGA) package, comprising thesteps of: providing a substrate that has a first surface and a secondsurface, wherein the substrate has a window-shaped aperture that extendsthrough the substrate from the first surface of the substrate to thesecond surface of the substrate; providing a stiffener; attaching asurface of a stiffener to the first surface of the substrate, wherein aportion of the surface of the stiffener is accessible through thewindow-shaped aperture; mounting a first surface of an IC die to theaccessible portion of the surface of the stiffener; mounting a firstsurface of a heat spreader to a second surface of the IC die; andcoupling a contact pad of the IC die to a ridge of the heat spreaderbetween the first surface and a second surface of the heat spreader witha wire bond.
 37. The method of claim 36, further comprising the step of:attaching a plurality of solder balls to the second surface of thesubstrate.
 38. The method of claim 36, further comprising the step of:configuring the second surface of the heat spreader to be attached to aprinted circuit board.
 39. The method of claim 36, wherein the contactpad is a ground contact pad, further comprising the step of: couplingthe ground contact pad to the heat spreader with the wire bond, whereinthe heat spreader operates as a ground plane.
 40. The method of claim36, wherein said heat spreader mounting step comprises the step of:providing a heat spreader that has a ridge around at least a portion ofits circumference such that an area of the first surface of the heatspreader is greater than that of the second surface of the heatspreader.
 41. The method of claim 36, wherein said stiffener providingstep comprises the step of: forming a central cavity in the surface ofthe stiffener, wherein the central cavity forms at least a portion ofthe accessible portion of the surface of the stiffener.
 42. The methodof claim 41 wherein said IC die first surface mounting step comprisesthe step of: mounting the IC die in the central cavity.
 43. The methodof claim 36, wherein said heat spreader mounting step comprises the stepof: providing a heat spreader that is substantially planar, wherein thesurface of the stiffener is substantially planar, wherein the accessibleportion of the stiffener is centrally located on the substantiallyplanar stiffener surface.
 44. The method of claim 36, wherein said ICdie first surface mounting step comprises the step of: mounting thefirst surface of the IC die to the stiffener with a first epoxy layer,wherein the heat spreader is mounted to the IC die with a second epoxylayer.
 45. The method of claim 36, wherein an area of the second surfaceof the IC die is greater than an area of the first surface of the heatspreader, further comprising the step of: configuring the heat spreaderto mount to the center of the second IC die surface.
 46. The method ofclaim 36, further comprising the step of: encapsulating the IC die andthe heat spreader.
 47. The method of claim 46, wherein saidencapsulating step comprises the step of: allowing the second surface ofthe heat spreader to be exposed through an encapsulant material.
 48. Themethod of claim 47, wherein said exposing step comprises the step of:exposing at least a portion of a circumferential surface of the heatspreader through the encapsulant material.
 49. The method of claim 36,wherein the substrate is a tape substrate, wherein said substrateproviding step comprises the step of: providing the tape substrate. 50.The method of claim 36, further comprising the step of: matching athermal expansion coefficient of the stiffener to the thermal expansioncoefficient of the heat spreader.